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Documentation: add note on altering data from frame change handlers 

Blender can crash while rendering, when scene data is changed from within
a `frame_change_pre` or `frame_change_post` callback function. This results
in bug reports like T60094, T67627, and T73530. Until this is properly
resolved, this limitation should be documented.

No functional changes.
This commit is contained in:
Sybren A. Stüvel 2020-02-11 10:52:37 +01:00
parent 757da61606
commit be8879718e
Notes: blender-bot 2023-02-14 08:08:56 +01:00 
Referenced by issue #77572, Reference image opacity value doesn't work with Blender 2.83+ (Intel HD 630 with Mesa 20, regression from D6729)
Referenced by issue #57646, Clip random marker placement in track/mask - not working in 2.8
Referenced by issue #56312, UI elements ignore keyboard layout changes (ex. Swapped CTRL / CAPS)
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doc/python_api/examples/bpy.app.handlers.2.py Normal file
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"""
Note on Altering Data
+++++++++++++++++++++
Altering data from handlers should be done carefully. While rendering the
``frame_change_pre`` and ``frame_change_post`` handlers are called from one
thread and the viewport updates from a different thread. If the handler changes
data that is accessed by the viewport, this can cause a crash of Blender. In
such cases, lock the interface (Render → Lock Interface or
:data:`bpy.types.RenderSettings.use_lock_interface`) before starting a render.
Below is an example of a mesh that is altered from a handler:
"""
def frame_change_pre(scene):
# A triangle that shifts in the z direction
zshift = scene.frame_current * 0.1
vertices = [(-1, -1, zshift), (1, -1, zshift), (0, 1, zshift)]
triangles = [(0, 1, 2)]
object = bpy.data.objects["The Object"]
object.data.clear_geometry()
object.data.from_pydata(vertices, [], triangles)